MCMP – Philosophy of Physics

Michael Krämer (RWTH Aachen) gives a talk at the MCMP Colloquium (23 October, 2013) titled "The (philosophy of the) Higgs". Abstract: I will review the physics of the Large hadron Collider LHC, including the recent the discovery of a Higgs particle, and the search for new physics beyond the Standard Model of particle physics. While the talk focusses on the physics, I will also mention various philosophical questions which are being addressed in the context of an interdisciplinary research group on the "Epistemology of the LHC".

Christian Wüthrich (San Diego) gives a talk at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013) titled "Time and space in causal set theory". Abstract: Causal set theory offers an elegant and philosophically rich, though admittedly inchoate, approach to quantum gravity. After presenting its basic theoretical framwork, I will show how space and time vanish from the fundamental picture it offers. The absence of space and time from the theory raises the serious question of whether such a theory can be empirically coherent at all, i.e, whether its truth would not undermine any justification we may heave for believing it. If it can be shown that spacetime re-emerges from the fundamental structure itn the appropriate limit, I will argue, then the threat of empirical incoherence is averted and it can be appreciated how space and time emerge from what there is, fundamentally, according to causal set theory. I shall close by sketching the prospects of the antecedent of this conditional claim.

Ioan Muntean (Fort Wayne) comments on Dean Rickles' "Dualities and the Physical Content of Theories" at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013).

Dean Rickles (Sydney) gives a talk at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013) titled "Dualities and Physical Content". Abstract: A duality expresses a reltionship between a pair of putatively distinct physical theories. Theories are said to be dualwhen they generate "the same physics", where same physics is parsed in terms of, e.g., having the same amplitudes, expectation values, observable spectra, and so on. Hence, theories related by dualities can look very different while making exactly the same predictions about observatle phenomena. Indeed, such theories can look sufficiently different that would-be interpreters would surely consider them to be representations of very different possible worlds. In this talk I will be concerned with the question of whether dualites reflect some deep aspect of reality, or whether they are simply a formal device that aids computations in difficult contexts (functioning in much the same way as a change of variables). This links quite naturally to problems of underdetermination, and also to the issue of what we mean by theory in such contexts. I defend a rather deflationary account fo the philosophical implications of dualities. Since the underdetermination is taken to apply to structurally distinct theories (that is, it is structure that is underdetermined), I will also consider whether dualities pose a problem for structural realists.

Radin Dardashti (MCMP/LMU) comments on Richard Dawid's "String Theory and Scientific Realism" at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013).

Richard Dawid (Vienna) gives a talk at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013) titled "String Theory and Scientific Realism". Abstract: T-duality, which is an important feature of string theory, implies that the string scale constitutes a minimal length scale: within the conceptual framework of the theory, no higher energy scale can provide new information. This in turn may be taken to suggest that, if ST is valid at its own characteristic scale, no new theories which become empirically distinguishable from string theory at higher energy scales should be expected to be found. The problem with this final theory claim hinges on the fact that it seems to beg the question by presupposing the conceptual framwork the finality of which it aims to establish. In the talk I aim at understanding significance an limitations of string theorys final theory claim based on the distinction between local and global limitations to scientific underdetermination. It will be argued that the final theory claim does carry some weight but cannot be understood independently from more general considerations about the scientific role of limitations to underdetermination. Some implications of final theory claims for the scientific realism debate are discussed in the second part of the talk.

Karen Crowther (Sydney) comments on Daniele Oriti's "Dissappearance and Emergence of Space and Time in Quantum Gravity" at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013).

Daniele Oriti (Potsdam) gives a talk at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013) titled "Disappearance and Emergence of Space and Time in Quantum Gravity". Abstract: We recall the hints for the disappearance of continuum space and time at microsopic scales, coming from classical and semi-classical gravitational physics. These include arguments for discreteness or for a fundamental non-locality, in a quantum theory of gravity. We compare how these ideas are realized in specific quantum gravity approaches, and focus in particular on the group field theory formalism, itself strictly related to other approaches, in particular loop quantum gravity. Next, we consider the emergence of continuum space and time from the collective behaviour of discrete, pre-geometric and non-spatio-temporal atoms of quantum space. After discussing the notion of emergence, with Bose condensates as one paradigmatic examples, and some specific cenceptual difficulties with the notion of emergent spacetime, we argue for spacetime as a kind fo cendsate, result of a phase transition, physically identified with the big bang. We then illustrate recent results, in the context of the group field theory framework, establishing a tentative procedure for the emergence of cosmological (homogeneous) spacetime and their effective quantum dynamics from fundamental, pre-geometric models. Last, we re-examine the conceptual issues raised by the emergent spacetime scenario in light of this concrete example.

Karim Thébault (MCMP/LMU) gives a talk at the MCMP Workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013) titled "Time Remains". Abstract: Even classically, it is not entirely clear how one should understand the implications of general covariance for the role of time in physical theory. On one popular view, the essential lesson is that change is relational in a strong sense, such that all that it is for a physical degree of freedom to change is for it to vary with regard to a second physical degree of freedom. This implies that there is no unique parameterziation of time slices, and also that there is no unique temporal ordering of states. At a quantum level this approach to general relativity is generally understood lead to a universe eternally frozen in an energy eigenstate. Here we will start from a different interpretation of the classical theory, and in doing so show one may avoid this acute "problem of time" in quantum gravity. Under our view, duration is still regarded as relative, but temporal succession is taken to be absolute. This is consistent with general covariance because it can be maintained only ba the addition of an arbitrary time parameter corresponding to the minimal temporal structure necessary for a succession of observations to be represented. This approach to the classical theory of gravity is argued to then lead to a relational quantization methodology, such that it is possible to conceive of dynamical observables within a theory of quantum gravity.

J. Brian Pitts (Cambridge) gives a talk at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013) titled "Real Change Happens in Hamiltonian General Relativity; Just Ask the Lagrangian (about Time-like Killing Vectors, First-Class Constraints and Observables)". Abstract: In Hamiltonian GR, change has seemed absent. Attention to the gauge generator G facilitates a neglected calculation: a first-class constraint generates a bad physical change in electromagnetism and GR, spoiling the constraints, Gauss's law or the momentum and Hamiltonian constraints in the (physically relevant) velocities. Only as a team G do first-calss constraints generate a gauge transformation. To find change, insist on Hamiltonian-Lagrangian equivalence. Change is ineliminable time dependence; in vaccum GR it is the absence of a time-like Killing vector field. Neglecting spatial dependence, invariantly something depends on time via Hamilton's equations iff there is no time-like Killing vector. According to Bergmann, reality is not confined to observables, defined as both gauge invariant (hence real) and economical (Cauchy data on space). Thus change can exist outside observables. Bergmanns lemma that observables have vanishing Poisson brackets for gauge transformations was imported by analogy to electromagnetism, neglecting the external vs. internal distinction and Hamiltonaian-Lagrangian equivalence. The resulting implausible Killing-type condition lacks the local examples required by Bergmann. Taking observables to be geometric objects (tensors, etc.) as usual in the 4-dimensional Lagrangian formalism makes the Poisson bracket of G with an observable the Lie derivative of a geometric object (on-shell): covariance, not invariance.